Plasma is made up of neutral gas, electron and ion. The neutral gas in plasma is activated by collision with the electron and ion to produce radical species(hereinafter referred to as "radicals"), which in turn emits light with a specific wavelength, mainly belonging to a radio-frequency wave or micro-wave. In the microelectronic industry, radicals have been known to play an important role for etching and deposition of thin films; and, therefore, studies on the spatial uniformity of the radicals have been actively carried out. In particular, since the spatial uniformity in a vacuum container used in the formation of plasma is crucial in good performance in the etching and deposition processes, there exists a continuous need to develop a method by which the radical distribution in plasma is measurable in an efficient and precise manner.
On the other hand, the actinometry employed in the etching process of thin films in the microelectronic industry, has been used for measuring the radicals in plasma. According to the actinometry, under a state that a small quantity of an inert gas called as an actinometer is added to such a reactive gas as SF.sub.6, CF.sub.4, Cl.sub.2, O.sub.2 and the like, distribution of radicals such as fluoride, chloride and oxide is measured by analyzing the intensity of the light emitted from the radicals and the actinometer. The aforementioned method has been well known as an optical emission actinometry(hereinafter referred to as "OEA").
The problem in measuring the radical distribution using the conventional OEA is explained in detail.
The radicals are excited by collision with the electrons in plasma to emit electromagnetic waves. At this time, the light intensity emitted from the radicals is not always proportional to the density of radicals, since the probability of the collision between the radicals and electrons is varied, as the energy distribution and density of electrons are changed. In this regard, the intensity of the light emitted from the radicals may be determined from the following equation (I) which is a function of the radical density and plasma parameters: EQU I.sub.Rad .varies.[Rad]n.sub.e k.sub.e (I)
wherein,
I.sub.Rad represents intensity of the light emitted from radicals; PA1 [Rad] represents density of the radicals; PA1 n.sub.e represents density of the plasma; and, PA1 k.sub.e is a coefficient depending upon the plasma parameters. PA1 I.sub.Act represents intensity of the light emitted from actinometer; and, PA1 [Act] represents density of the actinometer.
In the conventional OEA method, the radical distribution is measured by the following equation (II), in which proportional coefficient of the equation (I) is eliminated by employing the actinometer: EQU I.sub.Rad /I.sub.Act .varies.[Rad]/[Act] (II)
wherein,
As described above, the OEA method measures the radical distribution using the light intensity ratio between the radicals and the actinometer, grounded on the fact that the actinometer, like the radicals, emits light by the collision of electrons, while it does not affect the discharge characteristics of the radicals. At this time, the collision probability of the actinometer and electrons should be same as that of the radicals and electrons, as the energy distribution and density of the electrons are changed.
In the conventional OEA method, a spatial distribution of the radicals is measured by employing the Abel's transformation. To measure the spatial distribution of the radicals through Abel's transformation, an apparatus for focusing light should be provided at the exterior of a vacuum container and the spatial light distribution is measured by applying the distribution angles of the light projected in this way to Abel's transformation, after all spatially distributed light is projected in any one direction. Accordingly, the OEA method essentially requires a large window positioned at the vacuum container.
However, it was very difficult to install such a large window on the conventional plasma apparatus. Moreover, it was impossible to measure any spatial distribution which is not in axial symmetry, since the Abel's transformation is built under the assumption that all spatially distributed light is in axial symmetry. Also, the conventional OEA method causes the plasma to be fluctuated due to the use of actinometer, errors in measurement are essentially accompanied, which is grounded on a fact that the spatial distribution of the actinometer is not uniform, and the errors become greater due to the secondary errors caused by the Abel's transformation.